What are your feelings as a beginner in each language? Have you found any little design decisions that leave you scratching your head, or make you start cursing loudly at the computer? Have you built anything with them? If so, did you run into any issues that you feel were a result of the language?

I'm a bit of a hedonist, but if I were in your situation, I'd probably try to pick the one that was the most fun to work with. As you say, once you've started building, it won't be (or maybe for you it will be?) as easy to switch, and you probably don't want to figure out that you hate coding in whatever you've chosen while trying to finish your web app.

When in a variant of your situation (PHP/a PHP framework versus Python/Django), I chose Python. I'm not you, so we may have totally different tastes and may have come to a different conclusion given the facts. I've been pretty happy with my choice, though.